Sintered stainless steel metal alloy



SINTERED STAINLESC; STEEL METAL ALLOY Application January 13, 1954Serial No. 403,922

Claims. or. 29-152 No Drawing.

This invention relates to permeable, sintered, stainless steel articlesand to a method of making same.

There is a need in the art for metal articles having a desired uniformpermeability, high resistance to corrosion, and which retain theirstrength at relatively high temperatures. Metal filters are typicalexamples of such articles. The property of uniform permeability can bestbe obtained by a loose sintering process; and, in order to effect aproper sintering of metal powder without destroying this property in thefinished article, it is necessary to heat the metal to a partiallyliquid condition and .to hold it at this temperature until theindividual metal particles are properly fused together.

Austenitic stainless steel is a metal pro-eminently suited for thispurpose, but to our knowledge no one. has been able heretofore to makearticles having uniform permeability characteristic from this metal bycommercially-practicable procedures. Difficulty invariably isencountered because of the narrow solidus-liquidus range characteristicof this metal. For example, one powdered austenitic stainless steeladmirably suited for use in the manufacture of metal filters and thelike when tested produced weak, open specimens when heated to 2615" F.and formed a hard, dense specimen due to almost complete melting of themetal when heated to 2630 F. Manifestly, this range is too narrow foradequate furnace control, particularly in a commercial operation.

Carbon, sometimes used to broaden the solidus-liquidus range of steelalloys, has not been deemed practicable forthepurpose at hand because itis generally recognized in this art that carbon causes undesirable phasevariations in the steel and in general destroys essential properties-ofthe metal. The instant invention is concerned primarily with the use ofcarbon .in the production of uniformly permeable articles of austeniticstainless steel so as to broaden the solidus-liquidus rangeofthe metalsufficiently to assure proper furnace control without losing desirableproperties inherent in the metal which are essential to the.

article .and which are normally destroyed by the presence of carbon.

Specifically, it has been discovered that in the manufacture 10fsintered articles from austenitic stainless steel, carbon can beincorporated initially as an alloying element in the metal powder, ifconfined to certain critical limits, to broaden the liquidus-solidusrange so as to permit proper sintering of the particles under conditionsaffording adequate furnace control in a commercial operation; and then,either simultaneously with the sintering operation or after thesintering operation has been completed, this carbon can be removed bydecarburization due to the open permeable structure of the sinteredarticle without adversely affecting the desired properties of the metalto any significant or appreciable extent. After the decarburization stepthe finished article is, for all practical purposes, the same in carboncontent as though it had been made initially from metal particles freeor substantially free of carbon. The discovery and appreciation of thesecritical factors makes it possible for the first, time,

2,826,805 Patented Mar. 18, 1958 2 sosfar as we know, to make uniformlypermeable articles of high strength and resistancetocorrosion usingaustenitic steel powder by commercially practical procedures, and thismakes available to the art articles .of .this character havingproperties not heretofore available.

Austenitic stainless steel alloys ito which this invention pertains aregenerally definedin therart as iron-chromiumnickel steels which containnot less :than approximately seven percent nickel and not less thanapproximately seventeen percent chromium. It .is generally recognizedthat some variation .inthe proportions given are permissible withoutremoving the alloy from the category of austenitic stainless steel..Tests and experience indicate that the instant invention isnoperativein the case of all metal alloys identifiable in,.the metallurgical artas an austenitic stainless steel. Also, the austenitic stainless steelused in this invention may have .various minor amounts of other elementsconventionally .used .to affect the properties of, the metal. Typicalexamples of such other metals are molybdenum, titanium, .columbium, andsilicon.

,The shape of the individual particles comprising the austeniticstainless steel powder used in the manufacture of sintered articlesunder this invention is of some significance. Commercially usablearticleshaving a fair degree of uniformness of permeability can be madeusing metal particles of irregular shape. However, a marked and for somepurposes exeedingly importantimprovement in permeability characteristicsand particularly in the uniformness of permeability of the sinteredarticle is achieved by using a metal powder in which the individualparticles are spherical in form, It is not possible, of course, toproduce a metal powderin which eachparticle is a theoretically perfectsphere, but austenitic stainless steel powders have been produced inwhich all or substantially all the particles are approximately sphericalin form utilizing techniques disclosed in United States Patents Nos.2,460,992 and 2,460,993. Experience has shown that permeable sinteredarticles such as filters and the like made from powder producedaccordingto the teachings of these patents are unexpectedly and markedly superiorto similar but commercially usable articles made from other commerciallyavailable austenitic stainless steel powder.

As suggested, the amount of carbon in the steel must be confined withinrelatively critical limits. Specifically we have found that carbonshould be added in amounts greater than about 0.5% but not more thanabout 1.25% This carbon of course is added to and uniformly mixed inmetal as an alloying elementbefore the metal is atomized to produce thepowder so that the carbon is incorporated'in each individual metalparticle used in the sintering process to produce the finished article.The primary purpose of the carbon is to 'broaden the solidus liquidusrange of the metal sufiiciently to permit proper furnace control in themanufacture of the article and to produce sintered articles havinguniform permeability characteristics by commercially practical .methods.The carbon also serves in a secondary capacity to reduce the meltingpoint of the metal. This latter consideration is of some practicalimportance inasmuch as austenitic stainlesssteels melt at exceedinglyhigh temperatures in the order of 26002700 F. and this temperature isdestructive of vessels used to confine the metal during heating. Anysignificant drop in the melting temperature of the metal therefore addsa corresponding amount to the life of the heating vessel. Silicon alsomay be added as an alloying element in the steel to further reduce themelting temperature. Of course any amount of carbon in the steelbroadens the solidus-liquidus range to some extent but we have, foundvthat when added in amounts 1 less than 0.5% the carbon does not broadenthe solidusliquidus range sufliciently to permit adequate furnacecontrol under commercially practical conditions. On the other hand, ifcarbon is added in amounts greaterthan about 1.25% it producesundesirable phase variations in the steel which adversely affectproperties desired in the sintered article made from the powdered metal.For instance, excess amounts of carbon adversely affect the uniformpermeability characteristics of the article, and in the case of afilter, for example, this characteristic is of prime importance. Bestresults are obtained in any particular instance by employing carbon inan amount sufficient to increase the solidus-liquidus range of the metalto a point which just affords adequate furnace control.

The following are typical examples of austenitic staiuless steel powdersuseful in the practice of this invention:

Example I Percent Chromium 18.0 Nickel "8.0 Manganese 1.5 Silicon 5.0Carbon 0.6 \Iron 66.9 Example II c t Chromium 18.0 Nickel 8.0 Manganese1.5 Carbon 0.6 Iron 71.9 Example 111 Pe cen Chromium 18.0 Nickel 8.0Manganese 1.5 Silicon 2.5 Carbon 0.5 Iron 69.5

I Example IV Percent Chromium 18.00 Nickel 11.00 Molybdenum g 2.50Manganese 0.25 Silicon i 5.00 Carbon q 0.60 Iron 62.65

Example V i 7 Percent Chromium 18.0 Nickel 8.0 Manganese 1.5 Silicon 5.0Carbon 1.0 Iron 66.5

Example VI i Percent Chromium 20.0 Nickel 8.0 Manganese 0.5 Silicon 5.0Carbon 1.0 Iron 65.5

The procedure for making sintered metal articles from powdered alloysembodying the instant invention -is relatively simple. The powderedmetal is confined in any suitable manner as on' a supporting sheet or ina mold which negatively reproduces the desired article and heated in afurnace to a temperature sufiicient to partially liquefy the metalparticles. Regardless of how 'it is confined, the

powder is loose in the sense that there is no compacb ing of the powderor application of pressure to the powder before or during the heatingstep which results in sintering of the particles. The alloy identifiedin example I above reaches the desired partially liquid state at about2325 F., and the amount of carbon in this alloy broadens thesolidus-liquidus range thereof sufliciently to afford good furnacecontrol and to permit a sintered article having excellent uniformity ofpermeability to be produced commercially.

After the sintering operation has been completed, it is necessary toremove a large proportion and in some instances substantially all thecarbon in order to produce a satisfactory end product. This isaccomplished according to the present invention by a decarburizingprocedure. In some instances where the metal powder is exposed duringthe sintering step, decarburization may be carried on simultaneouslywith the sintering operation. However, in'situations Where the metalpowder is not exposed but is substantially entirely confined as in amold or the like during the sintering operation, it is necessary tocarry out' the decarburization as a separate step after the sinteringoperation has beencompleted and the article has been removed from themold.

The preferred decarburizing procedure is to heat the sintered article inan atmosphere of hydrogen for a length of time sufficient to remove thecarbon. If the article is heated during the idecarburization step to thesintering temperature, carbon is, of course, removed progressivelytherefrom as the temperature is raised; and as the carbon is removed,the melting point of the metal increases. Thus, in some instances, thesintered article can be heated to a temperature equal to or slightly inexcess of the sintering tempreature without adversely affecting thepermeability characteristics thereof to any appreciable or significantextent. The length of time required to remove all the carbon variesconsiderably, depending on the size and shape of the article, but thirtyminutes to two hours usually is sufiicient. The open, porous structureof the sintered article permits the hydrogen to permeate through allportions thereof and to remove carbon substantially completely from theinterior as well as the exterior portions. All but negligible amounts ofcarbon can be removed in this way, and in some instances final carboncontents in the order of 0.01% have been achieved.

Where conditions are such that decarburization can be carried outsimultaneously with the sintering operation the powder is placedinitially in the furnace and brought quickly to the sinteringtemperature in an atmosphere of hydrogen. Of course sintering occurs toproduce the desired article when the temperature enters theliquidussolidus range of the metal. As the temperature is brought tothis point, some carbon of course is removed, but as this part of theoperation is performed relatively quickly, not enough carbon is removedto narrow the liquidussolids range to any appreciable extent. As soon assintering .is accomplished the article is held at a temperature thatwill not adversely affect the sintered article but which will producethe desired decarburization.

In those situations where decarburization cannot be carried onsimultaneously with the sintering operation as where the metal powder isconfined in a mold during sintering so that the hydrogen gas does nothave access thereto, it is necessary to remove the sintered article fromthe mold and then place it in a furnace where it is exposed to hydrogenin order to eifect decarburization. This of course can be done by batchor continuous methods according tothe exigencies of the particularsituation.

' Having thusdescribed the invention, we claim:

1. The method of making uniformly permeable articles of austeniticstainless steel comprising the steps of loose sinteringaustenitic'stainless steel powder containing carbon prealloyed therewithin amounts from about 0.5% to about 1.25%, and decarburizingthe'sintered article thusformed by exposing the same to a decarburizingatmosphere. i

2. The method of making uniformly permeable metal articles having highresistance to corrosion and which retain their strength at relativelyhigh temperatures comprising the steps of sintering austenitic stainlesssteel powder containing from about 0.5% to about 1.25% carbon, andsimultaneously decarburizing the same by exposing said powder during thesintering operation to a decarburizing atmosphere.

3. The method of making uniformly permeable metal articles of austeniticstainless steel comprising the step of heating loosely confinedaustenitic stainless steel powder containing from about 0.5% to about1.25% carbon in an atmosphere of hydrogen to a temperature at which saidalloy is in a partially liquid state so as to sinter said powder andsimultaneouly decarburize the same.

4. The method of making uniformly permeable metal articles having highresistance to corrosion and which retain their strength at relativelyhigh temperatures comprising the steps of loose sintering austeniticstainless steel powder containing from about 0.5% to about 1.25% carbon,and then exposing the sintered mass of powder to a decarburizingatmosphere for a length of time sufllcient to remove substantially allof said carbon.

5. The method of making uniformly permeable articles of austeniticstainless steel com-prising the steps of loosely confining austeniticstainless steel powder containing from about0.5% to about 1.25% carbonand subjecting the same to a temperature within the solidus-liquidusrange of the steel for a length of time suflicient to sinter the same,and then exposing the sintered article to an atmosphere of hydrogen fora sutficient length of time to decarburize the same.

6. The method of making uniformly permeable metal articles of austeniticstainless steel comprising the steps of loose sintering austeniticstainless steel powder of a type wherein the individual particles of thepowder are essentially spherical in form and which contains from about0.5% to about 1.25% carbon, and decarburizing the sintered article toremove substantially all the carbon from the sintered powder mass byexposing the sintered article to a decarburizing atmosphere.

7. The method of making uniformly permeable metal articles having highresistance to corrosion and which retain their strength at relativelyhigh temperatures com prising the steps of sintering austeniticstainless steel powder containing from about 0.5% to about 1.25 carbon,and decarburizing said powder by subjecting said powder to adecarburizing atmosphere.

8. A new article of manufacture comprising a metal. article composed ofa sintered mass of austenitic stainless steel powder, said article beingthe product of claim 7 and containing negligible amounts of carbonapproaching 0.01% and being characterized by high resistance tocorrosion and oxidation, being capable of retaining a relatively highstrength at high temperatures, and being essentially readily uniformlypermeable throughout the entire mass thereof.

9. The method of making uniformly permeable metal articles of austeniticstainless steel comprising the steps of sintering austenitic stainlesssteel powder of a type wherein the individual particles of the powderare essentially spherical in form and which contains from about 0.5% toabout 1.25% carbon, and decarburizing said powder to removesubstantially all the carbon therefrom, by subjecting said powder to adecarburizing atmosphere.

10. Austenitic stainless steel powder for making unifornily permeablemetal articles containing not less than approximately seven percentnickel and not less than approximately seventeen percent chromium andcarbon pro-alloyed therewith in amounts from about 0.5% to about 1.25%,said powder being characterized by its broadened solidus-liquidus rangeand by having substantially all of its individual particles in sphericalform.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Kinzel et al.: The Alloys of Iron and Chromium, vol. II, 1sted., McGraw-Hill Book Co. (1940), pp. 368, 390.

Seymour: Powdered metal filters, The Mining Magazine (London), vol. 77(Oct. 1947), p. 207.

Goetzel: Treatise on Powder Metallurgy, vol. II, Inter- 5 sciencePublishers, Inc. (1950), pp. 389-399, 538,

'UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.2,826,805

March l8, 191 Robert L Probst (-21, 8-1.

'Column 1, line 31 characteristics for "in metal" head in the metalcolumn 4, lines 54, and 55,

for "to narrow the liquidus-solids range" .read to narrow theliquidus-solidus range Signed and sealed this 27th day of May 1958.

(SEAL) Attest:

KARL mm ROBERT C. WATSON Attesting Officer Conlnissioner of PatentsUNITED STATES PATEN'I 7 ICE CERTIFICATE OF CORRECTION I Patent No.2,826,805 March 18, 1958 Robert L. Probst et al. I It is herebycertifiedthat error appears in the printed specification of the abovenumbered patent requiring correction and that the said Letters Patentshould read as corrected below.

' Column 1, line 31, for "permeability characteristic" rea'dpermeability characteristics column 2, line 51, for "in metal" read inthe I metal --Y; column 4, lines 54, and 55, for to narrow the liquidussolids range" read to narrow the liquidus -solidus range A Signed andsealed this 27th day of May 1958.

Attest:

KARL mm ROBERT c. WATSON Attesting Officer Comnissioner of-Patents

1. THE METHOD OF MAKING UNIFORMLY PERMEABLE ARTICLES OF AUSTENITICSTAINLESS STELL COMPRISING THE STEPS OF LOOSE SINTERING AUSTENITICSTAINLESS STEEL POWDER CONTAINING CARBON PREALLOYED THEREWITH IN AMOUNTSFROM ABOUT 0.5% TO ABOUT 1.25%, AND DECARBURIZING THE SINTERED ARTICLETHUS FORMED BY EXPOSING THE SAME TO A DECARBURIZING ATMOSPHERE. 10.AUSTENITIC STAINLESS STEEL POWDER FOR MAKING UNIFORMLY PERMEABLE METALARTICLES CONTAINING NOT LESS THAN APPROXIMATELY SEVEN PERCENT NICKLE ANDNOT LESS THAN APPROXIMATELY SEVENTEEN PERCENT CHROMIUM AND CARBONPRE-ALLOYED THEREWITH IN AMOUNTS FROM ABOUT 0.5% TO ABOUT 1.25%, SAIDPOWDER BEING CHARACTERIZED BY ITS BROADENED SOLIDUS-LIQUIDUS RANGE ANDBY HAVING SUBSTANTIALLY ALL OF ITS INDIVIDUAL PARTICLES IN SPHERICALFORM